Studies on metabolic regulation are based on experiments designed to represent physiological energy states which are amenable to experimental manipulation. These studies are focused on furthering our understanding of metabolic interaction in various mammalian organs, the importance of inter-organ communication, communication via the bloodstream and, in particular, the role of energy transduction in the expression of this control. In order better to evaluate physiological regulation of metabolic processes, the studies entail experimental design of varying degrees of complexity. Isolated perfused muscle (hindquarter) preparations, isolated intact mitochondria (incubated with or without cellular enzymes and coenzymes to mimic the energetic states which obtain in the intact cell) isolated liver cells, and soluble extracts (containing glycolytic enzymes) from skeletal muscle (with and without phosphorylating mitochondria) are being exploited to exact multiple probes into the nature and regulation of cellular events.